1. Field of the Invention
The present invention relates to a radio transmitting/receiving device, particularly to an antenna malfunction detecting system for detecting an abnormality in an antenna system which is connected to a radio transmitting/receiving device.
2. Description of the Related Art
A conventional antenna malfunction detecting system disclosed in, for example, the patent application laid-open No. Hei 6-296168, as shown in FIG. 5, relates to a radio transmitting/receiving device of an automobile telephone system. To detect an abnormality in an antenna, a circulator 17 is inserted between an output of a final-stage amplifier 16 and an antenna 3. When there is an abnormality in the antenna, a reflected wave of an output signal of the final-stage amplifier 16 from the antenna is detected by a rectification circuit 21. When the reflected wave is large as a result of comparison with a preset reference voltage (REF), the final-stage amplifier 16 is controlled by a control circuit 23. Further, an alarm is given to a display circuit for indicating an occurrence of abnormality to a maintenance personnel.
Also, another conventional antenna malfunction detecting system is disclosed in the patent application laid-open No. Hei 5-136747, as shown in FIG. 6. In this technology, a directional coupler 2 is inserted between a radio transmitter 1 and an antenna 3. A traveling wave power from the radio transmitter 1 and a reflected wave power from the antenna 3 are detected and compared in a comparison circuit. Thereby, an increase of a reflected wave is detected, and an alarm is issued.
In the aforementioned conventional antenna malfunction detecting system for detecting a reflected wave, when there is a trouble in the antenna, the reflected wave occurring at a trouble point is detected. However, since a standing wave is generated between a reflection point and a wave source, the amplitude of the reflected wave varies depending on a position of a wave detector, and it might becomes maximum or minimum in accordance with the position of the wave detector for detecting the reflected wave. Therefore, the reflected wave cannot be detected precisely in some case. In this case, it is wrongly judged whether the antenna is normal or abnormal.